1. Field of the Invention
The invention relates to a projection apparatus which causes light from a light source to enter a reflective image display element, forms a light image by the reflected light, and projects the light image.
2. Description of the Related Art
FIG. 3 is a plan view showing a configuration of a general projector apparatus 10 of the Digital Light Processing (DLP) (registered trademark) type using a micromirror element.
In FIG. 3, a high-pressure mercury lamp 12 acting as a light source is arranged inside a reflector 11 whose interior surface is mirror-like finished. The high-pressure mercury lamp 12 is driven by an alternating-current high-voltage power supply, thereby producing high-intensity white light.
Light produced by the high-pressure mercury lamp 12 is taken out directly or is reflected by the interior surface of the reflector 11 and taken out as a beam. After having been reflected by a first mirror 13, the beam passes through a lamp lens 14 and is directed onto a rotating color wheel 15.
The color wheel 15 is such that a disk-shaped surface part that is rotated by a color wheel motor 16 is composed of a red (R), a green (G), and a blue (B) sector-shaped color filter. Therefore, when the color wheel 15 is rotated by the color wheel motor 16, white light passing through the color wheel 15 is extracted as R, G, B primary color light components in a time-division manner. That is, the frequency ranges of R, G, B primary colors are extracted cyclically in a time-division manner. The resulting light components are then output.
When passing through a light tunnel 17, the R, G, B primary color light components output from the color wheel 15 repeat diffuse reflection inside the light tunnel. After the distribution of luminous flux density is averaged, the resulting light components are reflected by a second mirror 19 via an illumination system lens 18. The reflected light components are then reflected by a third mirror 21 via an illumination system lens 20. Thereafter, the resulting light components are directed onto a micromirror element 23 via a field lens 22.
The micromirror element 23 is an image display element also called a Digital Micromirror Device (DMD) (registered trademark). The micromirror element 23 performs on/off control of the inclination angle of each of the micromirrors arranged in an array, for example, as many pixels as there are in a XGA (1024 pixels×768 pixels), to make a display, thereby forming a light image by its reflected light.
The light image formed by the reflected light from the micromirror element 23 is sent to a projector lens unit 24 via a field lens 22. The projector lens unit 24 has a lens configuration of three groups, a first lens group 24A, a second lens group 24B, and a third lens group 24C, starting from the object side. The projector lens unit 24 enlarges a light image formed by the micromirror element 23 and projects the enlarged image onto a screen (not shown) or a projection object on which an image is to be projected.
FIG. 4 is a diagram to explain a concrete configuration of the entry and reflection of light at the micromirror element 23 in the configuration of FIG. 3. In FIG. 4, an illumination system optical axis L shown by a broken line enters the array surface of micromirrors at an incidence angle of 24° to a normal line in the direction of a projection optical axis N shown by a solid line from a direction to which a 45° turn is made from a direction H in which each micromirror faces in an on operation at the array surface of the micromirrors.
FIGS. 5A and 5B are diagrams to explain a basic on/off operation of an individual micromirror constituting the micromirror element 23. Suppose a micromirror 23a is tilt-controlled at an angle of ±12° by an on/off operation.
FIG. 5A shows a state where a micromirror 23a has been tilt-controlled by an on operation, that is, at an angle of +12° (+A°). As shown in FIG. 5A, reflected light from the micromirror 23a travels along the projection optical axis N, the normal line of the micromirror element 23, and goes out toward the projector lens unit 24.
FIG. 5B shows a state where the micromirror 23a has been tilt-controlled by an off operation, that is, at an angle of −12°−A°). As shown in FIG. 53, reflected light from the micromirror 23a travels along an off beam optical axis D 48° (C.°) from the projection optical axis N, the normal line of the micromirror element 23, and is directed onto a light-absorbing member (not shown).
In the basic configuration shown in FIGS. 5A and 5B, if an on/off angle of the micromirror 23a is ±A°, an incidence angle of the illumination system optical axis L to the projection optical axis N for the projector lens unit 24 in the direction of normal line of the micromirror element 23 is B, and an angle of an off beam optical axis D to the projection optical axis N in an off operation is C, the following equations are satisfied: C=2B=4A.
Neither the beam directed onto the micromirror 23a nor the beam reflected by the micromirror 23a is parallel. They take the form of a cone, with the micromirror 23a at the vertex.
FIG. 6 shows an example of beam of each of incident light and outgoing light at the micromirror 23a in an on operation recommended by a manufacturer of the micromirror element 23. If the vertex angle of each of illumination system beam φL, an incident beam, and projection system beam φP, an outgoing beam, is 2β°, β is made less than or equal to angle A through which the micromirror 23a can move, which enables the amount of light from a light source to be used for projection effectively without the overlapping of illumination system beam φL and projection system beam φP and the occurrence of a so-called eclipse in the projected beam.
To make a projected image brighter, setting larger the radius of an illumination system beam input to the micromirror element 23 can be considered. In that case, the vertex angle of 2β° of each of illumination system beam φL and projection system beam φP can be considered to exceed an angle B between illumination optical axis L and projection optical axis N.
FIG. 7 shows a case where the vertex angle of 2β of each of illumination system beam φL and projection system beam φP is set so as to exceed an angle B between illumination optical axis L and projection optical axis N. When the vertex angle of each beam is designed to be greater than or equal to the recommended value, a loss part where illumination system beam φL and projection system beam φP overlaps in space occurs as shown by a downward-sloping hatching part in FIG. 7.
The loss part where the beams overlap is where the beams are partially blocked out by the third mirror 21 on the actual device. A part of incident light to the micromirror element 23 is missing. This gives rise to a phenomenon of the amount of light at one end of the finally projected image decreasing more than the remaining part.
As described above, when the vertex angle of each of illumination system beam φL and projection system beam φP is set larger to make an image brighter, the most part becomes a brighter image, but the distribution of light amount becomes nonuniform, which makes the image quality lower.
FIG. 8 is a diagram to explain a case where the incidence angle of illumination optical axis L is set larger than regular 24° to avoid the overlapping of two beams whose vertex angle is made larger.
In FIG. 8, the illumination optical axis L shown by a broken line is caused to enter the micromirror array surface at an incidence angle of 24°+α (α>0) to the projection optical axis N shown by a dashed-dotted line from a direction to which a 45° turn is made from the narrow side direction H of the micromirror element 23, the direction in which each micromirror tilts in an on operation. In FIG. 8, line LB represents a regular illumination system optical axis with an incidence angle of 24°. As described above, setting the illumination optical axis L outside the original illumination system optical axis LB causes the projection beam optical axis P to depart from a tangential direction N, the original projection optical axis, which enables illumination system beam φL and projection system beam φP to be prevented from overlapping in space.
Similarly, a method of eliminating a beam component resulting in a loss by causing the optical axis of an illumination beam to enter at a large incidence angle to the original illumination system optical axis has been considered (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2003-66366).
However, even if the optical axis of an illumination beam is caused to simply enter at an incidence angle larger than 24°, the original angle, from a direction to which a 45° turn is made from the narrow side direction of the micromirror element 23 as described in Jpn. Pat. Appln. KOKAI Publication No. 2003-66366, the distribution of a projection beam reflected by the micromirror element is left-right asymmetric. Therefore, the disadvantage is that the brightness of a projected image is also left-right asymmetric.